Fix ForeignKey queryset filters for cases where the to argument is a …

…string like 'app_label.model_name'.

mypy_django_plugin/django/context.py

@@ -4,7 +4,7 @@
 from contextlib import contextmanager
 from typing import TYPE_CHECKING, Any, Dict, Iterable, Iterator, Optional, Sequence, Set, Tuple, Type, Union
 
-from django.core.exceptions import FieldError
+from django.core.exceptions import FieldError, FieldDoesNotExist
 from django.db import models
 from django.db.models.base import Model
 from django.db.models.expressions import Expression
@@ -385,7 +385,25 @@ def solve_lookup_type(
             # Primary key lookup when no primary key field is found, model is presumably
             # abstract and we can't say anything about 'pk'.
             return None
-        return query.solve_lookup_type(lookup)
+        try:
+            return query.solve_lookup_type(lookup)
+        # This occurs when the following conditions are met:
+        # - model_cls._meta.abstract = True
+        # - part of the lookup is a foreign key defined on model_cls where the 'to' argument is a string
+        # On abstract models the 'to' parameter is only coalesced into the actual class whenever a subclass of it is
+        # instantiated, therefore it is never swapped out for abstract base classes.
+        except AttributeError:
+            pass
+        query_parts = lookup.split("__")
+        try:
+            field = query.get_meta().get_field(query_parts[0])
+            if len(query_parts) == 1:
+                return [], [query_parts[0]], False
+            sub_query = Query(field.related_model).solve_lookup_type("__")
+            entire_query_parts = [query_parts[0], *sub_query[1]]
+            return sub_query[0], entire_query_parts, sub_query[2]
+        except FieldDoesNotExist:
+            return None
 
     def resolve_lookup_into_field(
         self, model_cls: Type[Model], lookup: str

tests/typecheck/fields/test_related.yml

@@ -943,3 +943,29 @@
                     non_installed = models.ForeignKey(  # E: Cannot find model 'not_installed.NonInstalledModel' referenced in field 'non_installed'
                         "not_installed.NonInstalledModel", on_delete=models.CASCADE
                     )
+-   case: test_foreign_key_to_as_string
+    main: |
+        from myapp.models import Book, Publisher
+        publisher = Publisher.objects.create(name="Example Publisher")
+        book = Book.objects.create(name="Book", publisher=publisher)
+        Book.objects.filter(publisher=publisher)
+    installed_apps:
+        -   myapp
+    files:
+        -   path: myapp/__init__.py
+        -   path: myapp/models/__init__.py
+            content: |
+                from django.db import models
+                from django.db.models.query import QuerySet
+
+                class Publisher(models.Model):
+                    name = models.CharField()
+
+                class PrintedGood(models.Model):
+                    publisher = models.ForeignKey(to="myapp.Publisher", on_delete=models.CASCADE)
+
+                    class Meta:
+                        abstract = True
+
+                class Book(PrintedGood):
+                    name = models.CharField()